Method of making bonded metalceramic elements



June 2'7, 1967 P. w. MCMILLAN ETAL 3,328,145

METHOD OF MAKING BONDED METAL-CERAMIC ELEMENTS Original Filed March 5, 1962 I formed of a metal selected from the 3,328,145 Patented June 27, 1967 ice 3,328,145 METHOD OF MAG BONDED METAL- CERAMIC ELEMENTS Peter William McMillan and Brian Purd'am Hodgson, Stafford, England, assignors to The English Eiectric Company Limited, London, England, a British company Original application Mar. 5, 1962, Ser. No. 177,417, new Patent No. 3,236,610, dated Feb. 22, 1966. Divided and this application Aug. 9, 1965, Ser. No. 478,069

6 Claims. (Cl. 6533) This application is a division of our co-pending application Ser. No. 177,417, filed Mar. 5, 1962 and now Patent No. 3,236,610.

This invention relates to bonded metal-ceramic elements. Such elements are used, for example, to form printed circuit panels.

According to one aspect of this invention, a process for the manufacture of a bonded metal-ceramic element includes the steps of placing a fiat metal sheet component in a mould, placing a charge of molten glass, capable of controlled devi-trification on the flat metal sheet component, pressing the molten glass into the desired shape so as to bond the flat metal sheet component to the glass, and subjecting the bonded metal-ceramic element to a heat treatment so as to convert the glass into a predominantly crystalline ceramic material.

According to another aspect of the invention, a process for the manufacture of a bonded metal-ceramic element includes the steps of placing a metal component,

silver, silver-based alloy, copper, copper-based alloy and nickel, in a mould, placing on the metal component a charge of molten glass, capable of controlled devitrification, having a composition within the following range in percentage by weight and a coeificient of thermal expansion in the devitrified state in the range of 20-400 C. of between 135 and 175x10 Percent SiO 5 -80 PbO O-3O A1 0 0-3 ZnO 5-3 0 Li O 5-15 B 0 0-7 K 0 0-5 N320 0-5 P 0 1-3 pressing the molten glass into the desired shape to bond the metal component to the glass, and then subjecting the bonded metal-ceramic element to a heat treatment, so as to convert the glass into a predominantly crystalline ceramic material, the heat treatment including the steps of:

(i) Maintaining the bonded metal-ceramic element at a temperature Within the range 450 C.600 C. for at least minutes, and

(ii) Subsequently maintaining the bonded metal ceramic element at a temperature within the range 700 C-900 C. for at least 15 minutes.

In another aspect of the invention, a process for the manufacture of a bonded metal-ceramic element includes the steps of bonding a metal component, formed of a metal selected from the group consisting of copper, copper-based alloys, silver-based alloys and nickel, directly to a base consisting of a glass component, capable of controlled devitrification, having a composition within the following range in percentage by weight and having a coefficient of thermal expansion in the clevitrified state in the range -400 C. of between 135 and 175x10 group consisting of Percent Si0 50-80 PbO 0-30 A1 0 0-3 ZnO 5-30 Li O 5-15 B 0 0-7 K 0 0-5 N320 0-5 P 0 1-3 and then subjecting the bonded metal-ceramic element to a heat treatment so as to convert the glass into a pre dominantly crystalline ceramic material, the heat treatment including the steps of:

(i) Maintaining the bonded metal-ceramic element at a temperature within the range 450-600 C. for at least 15 minutes, and

(ii) Subsequently maintaining the bonded metalceramic element at a temperature within the range 700-900 C. for at least 15 minutes.

According to yet another aspect of the invention, a bonded metal-ceramic element consists of a metal component, formed of a metal selected from the group consisting of silver and silver-based alloys, bonded directly to a base consisting of a devitrified glass component.

A printed circuit panel may be made from a bonded metal-ceramic element hereinbefore referred to.

The term printed circuit is well known in the art, and does not imply that a printing operation is employed; the desired pattern of electrical connections may for eX- ample be formed by resist etching of and electrical conducting material which initially covers an insulating base.

In a preferred embodiment of the invention, the thermally-devitrifiable glass component is bonded to a copper or silver foil by a hot pressing operation.

A bonded metal-ceramic element manufactured in accordance with the invention possesses a number of advantages when formed into a primed circuit panel, as compared with commonly-used printed circuit panels having their bases formed of organic bonded laminates.

Examples of methods of manufacturing a me-talceramic element in accordance with the invention will now be described with reference to the accompanying drawings, of which:

FIG. 1 shows one method in accordance with the invention;

FIG. 2 shows a modification of the method of FIG. 1; and

FIG. 3 shows a second method in accordance with the invention.

A preferred range of compositions of the-rmally-devitrifia'ble glasses suitable for use in connection with the present invention is as follows:

Percent by weight SiO 50-80 PbO 0-3 0 A1 0 0-3 ZnO 5-30 Li O 5-15 B 0 0-7 K 0-5 N320 05 P 0 l-3 In certain preferred compositions, PbO is present within the range 0.5 to 30 percent by weight.

In addition to their suitability for bonding to a conducting metal in accordance with the invention, certain of the compositions in the above range have coeflicients of thermal expansion in the devitrified state in the range 20-400 C. of between and X 10* which make them particularly useful when bonded to copper and silver. It

'I) a will be understood that the invention is not necessarily limited to devitrifiable glasses in the above range and other devitrifiable glasses with suitable properties may be used.

Two preferred compositions have the following percentages by weight:

Composition A:

The thermal expansion coefiicients in the range 20-400 C. are 174 10' for the ceramic material formed from Composition A and 147 10 for that formed from Composition B. Composition A thus has a thermal expansion coefficient well matched to that of copper (183x10- and sufiiciently well matched to that of silver (203 X 10 and Composition B has a thermal expansion coefficient sufiiciently well matched to that of copper.

A further preferred composition has the following percentages by weight:

Composition C:

SiO 59.2 PbO 14.0 ZnO 13.1 Li O 9.0 K 2.0 P 0 2.7

The thermal expansion coefficient for the ceramic material formed from the latter glass is approximately 170 Where the metal component to be bonded is of copper or silver, the bond between the metal and the ceramic base is improved by applying a thin layer of nickel to the copper or silver. The nickel layer is preferably bet-ween 0.0001 and 0.0005 inch thick though it may he, say, 0.001 inch thick, and may be applied by electro-plating or, in the case where the metal is copper, by rolling a composite nickel-copper sandwich ingot down to the desired thickness.

A conducting metal component in the form of a thin metal foil is bonded to a glass plate in the following manner, reference being made to FIG. 1 of the drawings:

A steel or cast iron mould 10 of the desired shape to give the plate the desired contour is employed, and the metal foil 11 is placed in the mould and pressed into close contact with the base 12 of the mould. This ensures that good heat transfer takes place between the foil 11 and the mould 10, avoiding melting of the foil which may otherwise occur when the molten glass is brought into contact with it. The thickness of the metal foil is not critical, but is preferably between 0.002 and 0.005 inch.

A charge of molten glass 13 at a temperature ranging from 1000 C. to 1300 0, depending on its composition, is dropped on to the centre of the metal foil 11 and pressed into the desired thin fiat plate by means of a steel plunger 14. The bonded glass-metal plate thus formed is removed from the mould and transferred to an annealing furnace at a temperature of 450600 C. depending on the glass composition. Speed in these operations is essential both to prevent melting or distortion of the metal foil due to prolonged contact with the molten glass and to prevent excessive chilling of the pressed glass plate due to prolonged contact with the steel plunger, which might result in cracking of the glass. A complete cycle from the intro- 4 duction of the molten glass to the removal of the pressed plate from the mould should not occupy more than 60 seconds and is preferably accomplished more rapidly.

The glass-metal plate is then annealed at a temperature of 450600 C., depending on the composition of the glass, for a period of at least 15 minutes and is then cooled, at a rate not faster than 10 C. per minute, to room temperature. At this stage the glass is transparent and may be inspected to ensure that voids and other imperfections are not present, particularly at the glass-metal interface.

The plate is then subjected to a controlled heat treatment which is carried out, where the metal foil is copper, in an atmosphere of commercial nitrogen to prevent undue oxidation of the metal. Where the metal foil is silver, the heat treatment may be carried out in air.

The heat treatment, by which the glass is converted into a predominantly crystalline ceramic material, is as follows:

(i) The temperature is raised at a rate not exceeding 10 C. per minute and preferably at a rate of 3 to 5 C. per minute to a first holding temperature which ranges from 450 C. to 600 C. depending on the glass composition;

(ii) The first holding temperature is maintained for a period of not less than 15 minutes and preferably for a period of one hour;

(iii) The temperature is then further raised at a rate not exceeding 10' C. per minute and preferably at 3 to 5 C. per minute to a second holding temperature which ranges from 700 C. to 900 C. depending on glass composition;

(iv) The second holding temperature is maintained for a period of not less than 15 minutes and preferably for one hour;

(v) The temperature is lowered to room temperature at a rate not exceeding 10 C. per minute and preferably at 5 C. per minute.

If it is not required to examine the composite plates in the glass form, the annealing process can be omitted and the plates can be transferred straight from the moulding operation to a furnace held at the first holding temperature of the heat treatment cycle. At the completion of the heat treatment there is produced a ceramic plate to one face of which be conducting metal foil is bonded. This bonded metal-ceramic element may then have its metal foil etched by means which are well-known per se to afford the desired printed circuit.

Particular examples of the method of carrying out the invention will now be described. In the first examples glass of Composition A referred to above was bonded to copper foil both of 0.001 inch thickness and 0.002 inch thickness. In each case, the piece of foil was degreased and lightly pre-oxidized on one face by heating in a gas flame to the temperature of 600650 C. for a time not exceeding 20 seconds. By this means a very thin layer of black cupric oxide was formed on the surface of the copper. A mixture of suitable raw materials to give the desired glass composition was melted at 1300 C. in a refractory crucible containing a high proportion of zircon until the glass was homogeneous and free of bubbles. The temperature of the molten glass was then lowered to 1250 C. and it was allowed to reach equilibrium at this temperature. The pre-oxidized copper foil was placed in the bottom of a suitable steel mould and a suitable quantity of molten glass was poured on to the foil and rapidly pressed by means of a plunger. The composite glass-metal plate so formed was quickly transferred to an annealing furnace at a temperature of 480 C. This temperature was maintained for one hour and the furnace was then cooled to room temperature at a maximum rate of 5 C. per minute. After inspection the composite plate was heat-treated in a controlled atmosphere of nitrogen, to cause devitrification of the glass, as follows:

(i) The temperature was raised at a rate of 5 C. per minute to 500 C.;

(ii) The temperature of 500 C. was maintained for one hour;

(iii) The temperature was further raised at 3 C. per minute to 850 C.;

(iv) The temperature of 850 C. was maintained for one hour;

(v) The temperature was lowered to room temperature at a rate not exceeding 5 C. per minute.

It was found that the copper foil was firmly bonded to the devitrified glass plate by this process, and the bonded elements were found suitable for processing into printed circuits by the use of standard etching techniques.

In another particular example, pieces of copper foil 0.001 to 0.005 inch thick clad with a layer of nickel 0.0001 to 0.0002 inch thick by a rolling process were first degreased and then pre-oxidized by firing at 930 C. in wet nitrogen for a period of up to 5 minutes depending on the size of the foil. By this means an adherent layer of green nickel oxide was formed on the nickel surface, while the surface of the copper remained substantially free from oxide. A mixture of suitable raw materials to give glass composition C was melted at 1300 C. in a crucible made from a refractory ceramic containing a high proportion of zircon until the glass was homogeneous and free from bubbles. The temperature of the glass was then lowered to 1250 C. and the glass was allowed to reach equilibrium at this temperature. The pre-oxidized foil was placed in the bottom of a suitable steel mould as shown in FIG. 1 with the nickel surface uppermost and a suitable quantity of molten glass was placed on the foil and rapidly pressed by means of a plunger. The composite metal-glass plate so formed was transferred to an annealing furnace at a temperature of 480 C. This temperature was maintained for one hour and the furnace was then cooled to room temperature at a maximum rate of 5 C. per minute. After inspection and cutting to size, the composite plate was heat treated in a controlled atmosphere of nitrogen, to cause devitrification of the glass, as follows:

(i) The temperature was raised at a rate of 5 C. per minute to 500 (ii) The temperature of 500 C. was maintained for two hours;

(iii) The temperature was further raised at 23 C. per minute to 725 C.;

(iv) The temperature of 725 C. was maintained for one hour;

(v) The temperature was lowered to room temperature at a rate not exceeding 5 C. per minute.

It was found that the bonded metal-ceramic element thus produced showed good adhesion between hte nickelclad copper component and the devitrified glass component, the standard peel strength test giving a figure of up to 12 lb./inch. For printed circuit panels having a phenolic-resin bonded paper base as used hitherto, a peel strength of 6 to 8 lb./ inch is typical.

In the test referred to, a one inch wide strip of the foil is first peeled back over a short distance and bent at right angles to the base. An increasing load is then applied to the end of the foil, at right angles to the base, and the load required to start the foil peeling is noted, the result being expressed in lb./inch.

Pure nickel foil can also be bonded in exactly the same manner as described for the nickel-clad copper foil; the adhesion of this foil is also good, having a peel strength of approximately 10 lb./inch.

In a modification of the pressing process of FIG. 1, to be described with reference to FIG. 2, the metal foil is inserted into a mould incorporating a porous base 22. The base may consist of a steel plate drilled with large numbers of fine holes or it may be a sintered metal plate. The body of the mould 20 is hollow and is connected as shown at to a vacuum pump, so that when the metal foil 21 is placed on the porous base 22 it is sucked down into close contact with it. This ensures that the foil remains flat during the pressing operation and also ensures good thermal contact between the foil 21 and the base 22. The other steps in this process may be identical with those described above.

Another modification of the process, to be described with reference to FIG. 3, involves the use of a roller in place of the plunger described. This is specially applicable when plates are required of a larger area than can conveniently be made by pressing. The metal foil 31 is placed on the base 32 of the rolling apparatus, which again can be provided with means for sucking down the foil as described above. A stream of molten glass 33 is poured on to the foil immediately in front of a heavy steel roller 34 which moves forward as the glass is poured and rolls the glass into a thin sheet and at the same time presses the glass into close contact with the foil 31. Here again a rapid time cycle is employed and the bonded metalglass plate is rapidly transferred to an annealing furnace (or direct to the heat-treatment furnace) on completion of the rolling operation. The other steps in this process may be as described above.

What we claim as our invention and desire to secure by Letters Patent is:

1. In a process for the manufacture of a bonded metal/ glass-ceramic element wherein molten glass capable of controlled devitrification and consisting essentially of constituents selected from the following range in percentage by weight:

Percent SiO 54-60 PbO 0-14 ZnO 13-28 Li O 5-9 B 0 0-5 K 0 0-2 Nazo P 0 1-3 is applied directly to a metal component formed of a metal selected from the group consisting of silver, silver-based alloy, copper, copper-based alloy and nickel, said molten glass being formed into the desired shape of a glass component and at the same time being pressed into contact with said metal component to bond said glass component directly to said metal component to form a bonded metal/ glass element; and wherein said bonded metal/glass element is subjected to a first temperature within the range 450 C.-600 C. for at least fifteen minutes and subsequently heated at a rate of not more than 10 C. per minute to a second temperature within the range 700 C.- 900" C., said bonded element being maintained at said second temperature for at least fifteen minutes to devitrify the glass to a predominantly crystalline glass-ceramic and form said bonded metal/glass-ceramic element, said bonded metal/glass-ceramic element being allowed to cool from said second temperature at a rate of not more than 10 C. per minute, the devitrified glass-ceramic component having a coefficient of thermal expansion in the range 20 C.-400 C. of between and l0- the improvement which comprises the steps of:

(i) selecting a thin flat metal sheet as said metal component;

(ii) applying said molten glass to said thin flat metal sheet; and (iii) rapidly compacting said molten glass into a thin flat sheet of glass to form said glass component and at the same time pressing said molten glass into close contact with said thin flat metal sheet, while heat is simultaneously withdrawn from said thin flat metal sheet, to bond said glass component directly to said thin flat metal sheet without melting or distortion of said thin fiat metal sheet occurring.

2. In a process according to claim 1 the improvement further comprises the steps of:

(i) placing said thin flat metal sheet on a base of a rolling apparatus;

(ii) applying said molten glass to said thin fiat metal sheet as a stream of molten glass poured on to said thin fiat metal sheet immediately in front of a roller of said rolling apparatus; and

(iii) rapidly compacting said molten glass into said thin fiat sheet of glass and at the same time pressing said molten glass into close contact with said thin flat metal sheet, while heat is simultaneously transferred from said thin flat sheet metal to said rolling apparatus, to bond said glass component directly to said thin flat metal sheet by moving said roller forward as said stream of molten glass is being poured thereby quickly rolling said poured molten glass into said thin fiat sheet of glass.

3. In a process according to claim 1 the improvement further comprises the steps of:

(i) placing said thin flat metal sheet in a mould;

(ii) applying said molten glass to said thin fiat metal sheet by dropping a charge of said molten glass on to said thin flat metal sheet; and

(iii) rapidly compacting said molten glass into said thin flat sheet of glass and at the same time pressing said molten glass into close contact with said thin flat metal sheet, while heat is simultaneously transferred from said thin flat metal sheet to said mould, to bond said glass component directly to said thin fiat metal sheet by rapidly pressing said charge of molten glass into said thin fiat sheet of glass.

4. In a process according to claim 1 the improvement further comprises the step of subjecting said bonded metal/glass element to said first temperature within 60 seconds of first applying said molten glass to said thin flat metal sheet.

5. The invention according to claim 1 wherein heat is simultaneously withdrawn through the thin metal sheet.

6. A process for the manufacture of a flat bonded metal/glassceramic sheet including the steps of:

(i) placing a thin fiat metal sheet component in a mould;

(ii) preparing molten glass capable of controlled devitrification and having a composition within the following range in percentage by weight:

Percent SiO 5 0-80 PM) 0-3 0 ZnO 0-3 Li O 5-30 B 0 5-15 K 0 0-7 Na O 0-5 P 0 1-3 (iii) applying the molten glass to the thin flat metal sheet component, rapidly forming the molten glass into the shape of a thin flat sheet and at the same time pressing the molten glass into close contact with the thin flat metal sheet component, while heat is simultaneously transferred from the thin flat metal sheet component to said mould, so as to bond the glass directly to the thin fiat metal sheet component without melting or distortion of the thin flat metal sheet component occurring; and

(iv) subjecting the bonded metal/ glass sheet to a temperature within the range 450 C.-600 C. not more than seconds after first placing the molten glass on the thin fiat metal sheet component and maintaining the bonded metal/ glass sheet at that temperature for at least fifteen minutes, subsequently heating the bonded metal/ glass sheet at a rate not exceeding 10 C. per minute to a second temperature within the range 700 C.-900 C. and maintaining said second temperature for at least fifteen minutes to devitrify the glass into a predominantly crystalline glassceramic, and allowing the bonded metal/glass-ceramic sheet to cool at a rate of not more than 10 per minute.

References Cited UNITED STATES PATENTS 2,526,704 10/1950 Bair -59 2,889,952 6/ 1959 Claypoole 65-59 3,061,664 10/1962 Kegg 65-33 3,075,860 l/l963 Veres a- 65-33 3,220,815 11/1965 McMillan et a1 65-33 

1. IN A PROCESS FOR THE MANUFACTURE OF A BONDED METAL/ GLASS-CERAMIC ELEMENT WHEREIN MOLTEN GLASS CAPABLE OF CONTROLLED DEVITRIFICATION AND CONSISTING ESSENTIALLY OF CONSTITUENTS SELECTED FROM THE FOLLOWING RANGE IN PERCENTAGE BY WEIGHT: 